JAE516.fm Page 609 Thursday, June 14, 2001 4:11 PM
Journal of Animal
Ecology 2001
70, 609 – 620
Seed-crop size and eruptions of North American boreal
seed-eating birds
Blackwell Science, Ltd
WALTER D. KOENIG* and JOHANNES M. H. KNOPS†
*Hastings Reservation, University of California, Berkeley, 38601 E. Carmel Valley Road, Carmel Valley, California
93924, USA; and †School of Biological Sciences, University of Nebraska, 348 Manter Hall, Lincoln, NE 68588 – 0118,
USA
Summary
1. Eruptions occur when a species appears in unusually high densities within and often
outside of its normal range. We used 30 years of Christmas Bird Counts combined with
cone/seed-crop data on boreal coniferous trees, breeding bird surveys, and weather
records to test correlates of winter eruptions by 11 species of primarily boreal North
American seed-eating birds.
2. Eruptions of six species in eastern (red-breasted nuthatch Sitta canadensis L., blackcapped chickadee Parus atricapillus L., evening grosbeak Hesperiphona vespertina
Cooper, pine grosbeak Pinicola enucleator L., red crossbill Loxia curvirostra L. and
bohemian waxwing Bombycilla garrulus L.) and five species in western North America
(pine grosbeak, pine siskin Carduelis pinus Wilson, evening grosbeak, bohemian waxwing
and red-breasted nuthatch) correlated with a combination of large coniferous seed crops
in the previous year followed by a poor crop. Breeding population size in the year of the
eruption was also correlated positively with the event in two of the species. Eruptions in
these species apparently occur when large boreal seed crops resulting in high population
densities (via high overwinter survivorship and/or high reproductive success) are
confronted with a relatively poor seed crop the next autumn.
3. Eruptions of common redpolls Carduelis flammea L. and black-capped chickadees in
the west followed only large seed crops the previous year, suggesting that high density is
a more important factor leading to eruptions than seed crop failure. The opposite was
true for white-winged crossbills (Loxia leucoptera Gmelin) in the east, where eruptions
correlated only with poor current year seed crops. This was the only species supporting
the ‘seed-crop failure’ hypothesis as the sole cause of eruptions.
4. Purple finches Carpodacus purpureus Gmelin erupted following years when breeding
population densities were high for reasons apparently unrelated to the seed crop. Eruptions
of three species in both regions were uncorrelated with any of the variables tested.
5. We conclude that seed crops of boreal trees play a pivotal role in causing eruptions
for a majority of boreal species, usually through a combination of a large seed crop
resulting in high population densities followed by a poor seed crop rather than seed-crop
failure alone. Weather conditions were not a significant factor correlating with eruptions
in any of the species.
Key-words: boreal birds, population eruptions, population synchrony, seed production,
spatial autocorrelation.
Journal of Animal Ecology (2001) 70, 609–620
Introduction
Eruptive invasions of boreal seed-eating birds can be
© 2001 British
Ecological Society
Correspondence: Dr Walter D. Koenig, Hastings Reservation, 38601 E. Carmel Valley Rd., Carmel Valley, CA 93924,
USA. Tel: (831) 659 5981; fax: (831) 659 0150; e-mail:
[email protected]
dramatic events, with birds appearing in large numbers
in areas far outside of their normal range. In North
America, eruptions generally occur in the autumn
with birds returning in the spring, and appear to be
more closely allied to normal winter migration than to
nomadism (Hochachka et al. 1999). Unlike normal
migratory movements, however, they are irregular in
frequency and do not occur every winter.
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W.D. Koenig &
J.M.H. Knops
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
609–620
Two major hypotheses have been proposed to explain
eruptions. First, the ‘seed-crop failure’ hypothesis suggests
that the widespread failure of conifer and other tree
seed crops results in birds moving to sites potentially
outside of their normal range in search of food. The
alternative, ‘population density’ hypothesis suggests
that eruptions occur when populations are unusually
large due to high winter survival and/or an unusually
successful breeding season.
Numerous authors have proffered support for the
seed-crop failure hypothesis for one or more species,
including Reinikainen (1937), Formozov (1960), Svärdson
(1957), Davis & Williams (1957, 1964), Evans (1966),
Eriksson (1970) and, most impressively, Bock &
Lepthien (1976), who reported evidence that eruptions
of boreal seed-eating birds were inversely dependent on
what was apparently a circumboreally synchronized
pattern of seed crop fluctuations in boreal trees. Support
for the population density hypothesis has also been
substantial, with several authors concluding that high
densities are at least an important predisposing factor
for eruption events (Lack 1954; Ulfstrand 1963; Newton
1970, 1972; Van Gasteren et al. 1992).
The critical prediction of the seed-crop failure hypothesis is that there should be a negative correlation
between the current seed crop and eruption events.
Testing the population density hypothesis is not as
straightforward. Because the occurrence of eruptions
is generally judged by the presence of relatively large
numbers of birds in and beyond their normal wintering
range, any direct relationship between eruptions and
winter population density will invariably be circular.
Consequently, we assume that large seed crops are
likely to result in relatively high population densities via
increased survivorship and/or reproductive success, and
test the indirect prediction that population eruptions
should be preceded by a large seed crop the previous
year. In some cases, high population densities may be
directly detectable by independent estimates obtained
during the previous spring, and might also be suggested
by environmental conditions favourable for breeding.
The seed-crop failure and population density hypotheses are not mutually exclusive. Indeed, it is likely
that eruptions might be most dramatic when a large
seed crop leading to high population densities is followed by a seed crop failure forcing the unusually large
numbers of birds present to search for food in areas
beyond their normal wintering range, in which case
eruptions result from a combination of the two hypotheses (Lack 1954; Newton 1972).
Our analyses extend the previous work of Bock &
Lepthien (1976). There are at least two reasons why
such a reanalysis is appropriate. First, thanks to efforts
by the Laboratory of Ornithology at Cornell University
and the National Biological Service, many more years
of Christmas Bird Count (CBC) data are now available
than those computerized painstakingly by Bock &
Lepthien (1976). Similarly, both environmental data
and data from the North American Breeding Bird
Survey (BBS) are now readily available. Secondly, Bock
& Lepthien (1976) did not make a concerted effort to
obtain seed production data by boreal trees, and it is
thus desirable to re-examine their conclusions in light
of a more extensive (and independent) database.
Methods
  
We considered 11 species of primarily boreal seed-eating
birds including the evening grosbeak, pine grosbeak,
purple finch, red crossbill, white-winged crossbill,
hoary redpoll (Carduelis hornemanni Holböll), common redpoll, pine siskin, bohemian waxwing, redbreasted nuthatch and black-capped chickadee. The
most commonly eaten items in the winter diet of these
species are summarized in Table 1. Although they vary
considerably in their dietary preferences, all are
dependent primarily on seeds in the winter, usually but
not always of boreal trees, with the exception of blackcapped chickadees, which eat a mixture of seed and
insects in winter.
CBC data on these species spanning 30 years between
the winters of 1959–60 and 1988–89 were downloaded
from the database maintained by the National Biological
Service. Files were used as given except that counts that
did not overlap in time and were within 3 min of both
latitude and longitude were assumed to be continuations of the same site and combined.
For all species × site combinations, birds per party
hour were determined for each year the count was
performed. These values (x) were log-transformed
(log(x + 1)). This procedure tends to normalize the
distributions and makes intuitive sense: the difference
between observing 1 and 10 birds per party hour (pph) is
more comparable to the difference between observing
10 and 100 birds pph than the difference between observing 91 and 100 birds pph. In all, we analysed data from
31 445 counts conducted at 2350 sites spread out over all
but a handful of states and Canadian provinces (Fig. 1).
We then removed any long-term trend in numbers at
each site by using the residuals from a regression of
(log-transformed) birds pph on year (Koenig 1999).
As an index of eruptions, we then determined the
proportion of sites in each year at which more than the
expected number of birds of each species (after removing the long-term trend) was counted (i.e. the residual
of the number of birds pph on year was positive). Data
were divided into two geographical regions separated
by the eastern edge of the Rocky Mountains (hereafter
‘eastern’ and ‘western’ North America; Fig. 1), because
of the distinct differences between eastern North America
and the western montane regions found by Bock &
Lepthien (1976). Analyses were performed within regions
(i.e. western seed crop vs. eruption index in the west) to
detect north/south movements. However, at least some
boreal species, including common and hoary redpolls
(Troy 1983), have been documented making west/east
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Table 1. Dietary preferences of the species during the winter. All refer to seeds except where indicated
Species
Diet
Source
Evening grosbeak
Maple (Acer), dogwood (Cornus), pine (Pinus), wild cherry
(Prunus), cedar (Juniperus)
Pine, snowberry (Symphoricarpos), blackberry (Rubus)
Martin, Zim & Nelson 1951
Pine grosbeak
Purple finch
Red crossbill
White-winged crossbill
Hoary redpoll
Common redpoll
Pine siskin
Bohemian waxwing
Red-breasted nuthatch
Black-capped chickadee
Elms (Ulmus) various fruits
Pines, firs (Abies)
Spruce (Picea), pines, larch (Larix)
Alder (Alnus), birch (Betula), willow (Salix)
Ragweed (Ambrosia), alder, birch, smartweed (Polygonum)
Filaree (Erodium), pines
Mountain ash (Sorbus), cedar, hawthorn (Crataegus)
various fruits
Pine, fir, spruce
Insects and insect eggs; pine seeds
Martin et al. (1951)
Martin et al. 1951
Martin et al. 1951; Bent (1968)
Martin et al. 1951; Lack 1954
Bent 1968
Martin et al. 1951
Martin et al. 1951
Lack 1954; Bent (1950)
Martin et al. 1951; Bent (1948)
Martin et al. 1951
density was calculated for the two geographical regions
of North America and for both < 50° and > 50°N
latitude. Insufficient data were available for either hoary
redpolls or bohemian waxwings for analysis.
   
Fig. 1. Geographic locations of Christmas Bird Count (CBC)
sites and of coniferous tree seed-crop data used in the
analyses. Also marked is the approximate eastern edge of the
Rocky Mountains, used as the dividing line between eastern
and western North America, and 50°N latitude, used as the
cut-off for the CBC sites and most of the other analyses. +,
Birds; d, seed crop.
movements, and thus we also performed analyses across
regions (i.e. western seed crop vs. eruption index in the
east). In order to focus on years in which relatively large
numbers of birds wintered outside their usual more
northern range, we restricted the CBC data to sites
< 50°N latitude.
  
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
609– 620
As estimates of density in the spring prior to eruptions,
we used data collected between 1968 and 1996 for the
11 focal species from the North American Breeding
Bird Survey (BBS) programme (ftp://ftp.nbs.gov/pub/
data / bbs). Results from 8276 sites, representing the
number of birds seen or heard during a standardized
census at the site during the spring breeding season,
were log-transformed and standardized and the proportion of sites yielding greater than the expected
(log-transformed) number of birds of the focal species
in a particular year (after removing any long-term
trend at the site) determined. This index of relative
Data on boreal tree seed and/or cone production were
obtained from Koenig & Knops (2000). Only sites with
at least 5 years of data were used. In all, we used 163
data sets on seven genera of North American coniferous
boreal trees from 28 different sources, yielding a total
of 1923 years of data between 1900 and 1993 (Fig. 1).
Genera used in the analyses included Abies, Larix, Picea,
Pinus, Pseudotsuga, Tsuga and Thuja. Analyses were
conducted by combining all coniferous genera and for
the genus Pinus by itself. Insufficient data were available to separate out any of the other genera of boreal
trees.
Seed and cone production data were originally
collected in a wide variety of ways. In order to allow
comparisons across studies, they were standardized as
follows. If the seed production data were categorical,
we ranked the categories in order of increasing crop
size, giving the highest category a 10, the lowest category
a 0, and making the difference between all intermediate
categories equal. For example, if only three categories
were used (i.e. good, fair and poor), years when the
crop was rated as good were given a 10; those in which
the crop was rated as fair were given a 5; and those in
which it was rated as poor were given a 0. Categories
were divided more finely, but still equally, if more than
three categories were used. Thus, if six categories were
reported (i.e. excellent, very good, good, fair, poor, very
poor), ratings were assigned the values 10, 8, 6, 4, 2 and
0, respectively. If values presented were interval or
ratio-level data, such as the actual counts or number of
acorns falling in traps, values were log-transformed.
As with the bird census data, we then determined the
proportion of sites for which the seed crop in a particular
year was above the overall mean, dividing the data into
eastern and western North America as above. Analyses
JAE516.fm Page 612 Thursday, June 14, 2001 4:11 PM
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W.D. Koenig &
J.M.H. Knops
also included the proportion of sites with above-average
seed production in the prior year (current year seed
production lagged 1 year).
These data represent a nearly comprehensive synthesis
of all published conifer seed or cone production data
available for North America (Koenig & Knops 2000).
Even so, the data are far more limited and of more
variable quality than the other data sets used here. In
particular, most data sets were taken in the montane
regions of the western United States, with few from more
northern boreal zones or from eastern North America
in general (Fig. 1).
Despite these shortcomings, these data provide the
best opportunity for directly testing the seed-crop
failure hypothesis on such a large scale. One reason for
this is that the seed-crop data were all taken independently of the bird data and thus are not potentially biased
by those involved in obtaining the data, in contrast to
many of the far fewer data sets used in the earlier analysis
by Bock & Lepthien (1976). Furthermore, previous
work (Koenig & Knops 1998, 2000) has documented a
significant degree of synchrony in seed production among
these genera of conifers extending over geographical
distances up to 2500 km, compensating in part for the
highly patchy distribution of seed-crop data available
outside western montane North America. As with the
other data sets, we divided sites into those above and
below 50°N latitude.
 
Monthly rainfall and mean temperature were obtained
from the National Oceanic and Atmospheric Administration website (ftp://ftp.ncdc.noaa.gov/pub/data/ghcn).
Two time periods were considered: winter, from 1 October
to 31 December of the current year, and summer, from
1 April to 31 July of the current year. Variables considered included total rainfall and mean daily temperature.
As with the previous data, values were standardized by
using the residuals of a regression on year (within sites) in
order to eliminate long-term trends. We then determined
the proportion of sites at which the total precipitation or
mean temperature was greater than the expected value,
dividing the data into eastern and western North America
as defined above. We also divided data into sites > 50°N
latitude and < 50°N latitude with the goal of including
effects potentially attributable to either conditions in
the more northern breeding ranges or the more southern
wintering ranges of the species considered.
Overall, sample sizes ranged from 870 to 1314 sites in
the east and between 472 and 1132 sites in the west,
depending on the year. Sample sizes were smaller for
sites > 50°N latitude, usually between 40 and 80 sites in
the east and 100 – 300 sites in the west.
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
609–620
 
As an index of annual variability in numbers, we calculated the coefficient of variation (CV = SD × 100/mean)
in the proportion of sites reporting greater than the
expected number of birds in a particular year. For a
comparison with related species not usually considered
to erupt, we contrasted these data with annual variability of house finches (Carpodacus mexicanus Müller)
(not measured in the east, where numbers increased
dramatically over the time period considered), American
goldfinches (Carduelis tristis L.), lesser goldfinches
(C. psaltria Say), white-breasted nuthatches (Sitta
carolinensis Latham) and chestnut-backed chickadees
(Parus rufescens Townsend) (not present in the east).
Because of limitations of the seed production data, it
proved unfeasible to perform multivariate analyses with
all the relevant variables considered simultaneously.
Consequently, we calculated Spearman rank correlations between the eruption index and the independent
variables separately. Because of the limitations of the
seed-crop data discussed above, we set α = 0·05 (twotailed) for tests involving these data. With analyses
performed on each species for eastern and western North
America both above and below 50°N latitude for conifers
combined and the genus Pinus (unavailable > 50°N
latitude in the east), a total of 308 tests involving the
seed-crop data yielded an expected three significant
results at the 0·01 level and an additional 12 at the 0·05
level by chance alone. This complication is considered
further below.
For comparative purposes, we also report correlations producing P < 0·05 for tests involving the BBS
data. With 72 such tests (two for each species in eastern
and western North America except for hoary redpolls
and bohemian waxwings), four of these are expected
to be significant by chance alone. Given their more
exploratory nature, we set α = 0·01 for tests involving
the weather variables. With a total of 352 correlations,
four are again expected to be significant by chance at
this level.
Results
 
The CV in the proportion of sites reporting greater
than expected numbers of birds in a particular year
ranged between 27·8% (purple finches in the east) and
181·3% (hoary redpolls in the west), averaging 62·9%
(Table 2). As expected, these values were considerably
greater than those for the five non-eruptive species
(Mann–Whitney test; Z = 3·9; N = 8, 22; P < 0·001),
annual CVs for which were all < 32·1% and averaged
only 19·4% (data not shown). In general, correlations
in the proportion of sites reporting greater than expected
values in the east vs. the west were positive but not
significant, exceptions being for winter precipitation
and five of the 11 species of birds, especially common
redpolls, pine siskins, bohemian waxwings and redbreasted nuthatches (Table 2).
Examples of the distribution of sites reporting greater
than expected numbers of individuals by year are graphed
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eruptions
Table 2. Coefficient of variation and Spearman rank correlations between the proportion of sites reporting greater than the
standardized average in a particular year in the eastern (east of the Rocky Mountains) vs. western North America, all south of
50°N latitude. CVs of temperature are indetermimate. P-values < 0·05 are in bold
CV (%)
Species
East
Winter population size of birds (1959 – 88)
Evening grosbeak
56·0
Pine grosbeak
75·7
Purple finch
27·8
Red crossbill
79·7
White-winged crossbill
78·5
Hoary redpoll
94·1
Common redpoll
68·0
Pine siskin
55·2
Bohemian waxwing
66·8
Red-breasted nuthatch
41·3
Black-capped chickadee
30·6
Seed crop (1958 – 92)
Coniferous seed crop
60·2
Pine seed crop
55·0
Weather (1958 – 92)
Summer temperature
–
Summer precipitation
29·3
Winter temperature
–
Winter precipitation
38·3
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
609– 620
West
rs
P-value
N years
53·1
53·5
30·3
41·0
125·9
181·3
69·2
34·6
46·1
36·0
31·9
0·33
0·26
0·12
0·42
0·25
0·32
0·71
0·61
0·66
0·44
– 0·18
0·07
0·16
0·52
0·02
0·18
0·09
0·001
0·001
0·001
0·015
0·34
30
30
30
30
30
30
30
30
30
30
30
54·0
52·2
0·16
0·09
0·46
0·75
23
17
35·9
–
33·3
0·00
0·18
– 0·12
0·45
0·99
0·30
0·50
0·007
35
35
35
35
–
for 21 of the years in Fig. 2 for the common redpoll,
one of the rarer species, and in Fig. 3 for red-breasted
nuthatches, one of the more common species. Although
the overall abundance of the latter makes contrasting
some of the years difficult, eruption years, including
1969 and 1985, stand out from years in which relatively
few birds were counted, such as 1970 and 1988. Graphs
of the proportion of sites reporting greater than expected
numbers of birds for eight of the species are presented
in Fig. 4, while examples of the proportion of sites
reporting greater than average seed crops, higher summer
temperatures, and greater than average summer rainfall
are graphed in Fig. 5.
Significant correlations between the eruption index
of the 11 species and the seed crop, BBS data and
environmental variables measured within the same
geographical region are summarized in Table 3. Only
two species (red crossbills and hoary redpolls) failed to
correlate with any of the variables. Eruptions of eight
of the 11 species correlated with either the previous
year’s seed crop, the current year’s seed crop, or (in six
of the species) with both in one or both geographical
regions. Several of the more northern species, including
white-winged crossbills and bohemian waxwings,
correlated more strongly with seed crops > 50°N
latitude than < 50°N latitude, but this was variable.
Two examples of species for which eruptions correlated
with both the prior and current year’s seed crop are
plotted in Fig. 6.
In contrast to the strong relationships with the seed
crop, no significant correlation with any of the environmental variables emerged. Eruptions of five species
correlated with breeding density, only one more than
expected by chance. With one exception the correlation
was positive; that is, eruptions were preceded by greater
than average numbers of sites reporting the species in
the prior spring.
Unsurprisingly, far fewer significant correlations were
found when comparing eruptions in one geographical
region with the seed crop, density and weather of the
other geographical region (Table 4). However, those few
significant correlations that did emerge mainly reinforced
the relationships reported in Table 3 (i.e. pine grosbeak,
purple finch, white-winged crossbill, bohemian waxwing
and red-breasted nuthatch) with two exceptions: eruptions of pine siskins in the west correlated with high
breeding densities > 50°N latitude in the east and, more
interestingly, eruptions of red crossbills in the east
correlated both with large western seed crops in the
previous year and poor western seed crops in the
current year.
There were no strong differences between species in
eastern and western North America. In both regions,
eruptions of seven of the 11 species were related to the
seed crop. In both the east and the west, five species
correlated with both good seed crops in the previous year
and poor seed crops in the current year (usually within
the same region, except for red crossbills in the east).
Discussion
As expected, the eruptive species analysed here exhibited greater annual variability in numbers than a small
comparison group of closely related noneruptive species.
Although synchrony in eruptions usually extends over
large geographical areas (W. D. Koenig, unpublished
JAE516.fm Page 614 Thursday, June 14, 2001 4:11 PM
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W.D. Koenig &
J.M.H. Knops
Fig. 2. Sites for which the number of common redpolls counted per party hour between winter 1968 and 1988 was greater than
the number expected for that site after eliminating long-term trends. The approximate geographical boundary of North America
is overlaid on the graph for 1968. Data based on Christmas Bird Counts.
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
609–620
data), eruptions east of the Rocky Mountains were
strongly correlated with those in the west for only
four species (common redpoll, pine siskin, bohemian
waxwing and red-breasted nuthatch). For the other
species, the proportion of sites reporting greater than
the expected number of birds in the two regions were
usually only weakly positive.
Results of correlations with winter densities are consistent with the hypothesis that eruptions of many boreal
seed-eating birds are related to the size of the seed
JAE516.fm Page 615 Thursday, June 14, 2001 4:11 PM
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eruptions
Fig. 3. Sites for which the number of red-breasted nuthatches counted per party hour between winter 1968 and 1988 was greater
than the number expected for that site after eliminating long-term trends. Data based on Christmas Bird Counts.
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
609– 620
crop of coniferous trees (Table 5). Despite the large
number of tests performed, these relationships are highly
unlikely to be due to chance. In all, 30 significant correlations (10 at the 0·01 level) with the seed crop emerged,
far more than the 15 (three at the 0·01 level) expected
by chance. More convincingly, every one of the 17
significant correlations with the previous seed crop was
positive, while all 13 of the significant correlations
JAE516.fm Page 616 Thursday, June 14, 2001 4:11 PM
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W.D. Koenig &
J.M.H. Knops
Fig. 4. The proportion of sites reporting greater than expected numbers of birds after eliminating long-term trends for both
eastern and western North America (as defined in Fig. 1) for eight of the 11 boreal seed-eating species considered. Data based on
Christmas Bird Counts. e, East; m, west.
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
609–620
with the current seed crop were negative, values significantly different from the 50 : 50 ratio expected if
these results were due to chance (χ 12 > 13, both P < 0·001).
Conclusions are identical, although less extreme,
considering each species as a single datum. Thus, despite
the problem of multiple comparisons, these analyses
provide good evidence that the seed crop has a strong
effect on eruptions of most boreal seed-eating birds.
For two species in the east and two different species
in the west, eruptions correlated with a combination
of large seed crops followed by poor seed crops, with
additional independent evidence for relatively large
densities of breeding birds prior to the eruption. For
these species, eruptions apparently occur when good
seed crops in the autumn result in relatively high survivorship and thus high densities of birds the following
spring (with potentially high reproductive success as
well), that then experience a poor seed crop during the
subsequent autumn. Given that good seed crops of boreal
trees are generally followed by relatively poor seed
crops (Koenig & Knops 2000), this pattern of seed-crop
production is likely to be fairly common.
JAE516.fm Page 617 Thursday, June 14, 2001 4:11 PM
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Fig. 6. Scattergrams of the eruption indices for (a) evening
grosbeaks and (b) red-breasted nuthatches in western North
America vs. the proportion of sites for which the pine cone/
seed crop was greater than average in both the prior and
current years. For both species, eruptions are correlated with
good seed crops in the previous year and poor seed crops in
the current year. m, Previous year; e, current year.
Fig. 5. The proportion of sites reporting a larger than average
seed crop, higher than expected summer temperatures and
greater than expected summer rainfall in both eastern and
western North America. e, West; m, east.
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
609– 620
For four species in the east and three in the west, we
detected correlations between eruptions and both large
seed crops followed by poor seed crops, but with no
independent evidence for high population densities
prior to the eruption. In these populations it is possible
that the critical effect of the good seed crop was not
on overwinter survivorship, but rather on reproductive
success the following spring. Once again, relatively large
numbers of birds would then erupt when faced with a
poor seed crop the next autumn. Indirect evidence for
this scenario would include the presence of large
proportions of immatures during eruption events, a
finding for which there is considerable evidence, particularly in European species (Lack 1954; Van Gasteren
et al. 1992; Riddington & Ward 1998).
Eruptions of two species in the west correlated only
with large seed crops the previous year, suggesting that
for these species high densities alone may be a more
important predisposing factor leading to eruptions than
seed-crop failures. The opposite was true for white-winged
crossbills in the east, whose eruptions correlated only
with seed-crop failures during the year of the eruption.
Eruptions of purple finches in both the east and the
west correlated with high breeding densities that were
apparently unrelated to the seed crop. No significant
correlation with any relevant variable was found for
three species in the east and three in the west, only one
of which (hoary redpoll) was common to both regions.
Thus, the only population for which the seed-crop
failure hypothesis by itself was supported was the whitewinged crossbill in the east. The population density
hypothesis by itself was also supported in only three
species, including purple finches, whose population
densities were apparently unrelated to the seed crop,
and common redpolls and black-capped chickadees in
the west, for which eruptions were correlated with a
large seed crop in the previous year. The majority of
species (six in the east, five in the west) support the
combination of these factors being critical to eruption
events, more or less as envisioned by Lack (1954) and
Newton (1970). Our results provide no support for
hypotheses relating eruptions directly to environmental
conditions, such as relatively mild winters leading to
high overwinter survivorship (Van Gasteren et al. 1992),
severe winters resulting in abandonment of normal
winter ranges or warm conditions during the breeding
season resulting in high reproductive success.
These results explain the variability in conclusions
reached by previous workers studying one or more of
these species usually on a smaller geographical scale.
For about half the populations and more than half
the species considered here, eruptions appear to result
from what amounts to a combination of both the
seed-crop failure and population density hypotheses.
JAE516.fm Page 618 Thursday, June 14, 2001 4:11 PM
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W.D. Koenig &
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Table 3. Significant Spearman rank correlations between the eruption index for all 11 species and the overall seed crop of
coniferous trees and of the genus Pinus only in both the current and prior years, mean winter (1 October–31 December) and
summer (1 April–31 July) temperature, winter and summer precipitation, and the density of breeding birds the previous spring
based on the Breeding Bird Surveys. All variables are expressed as the proportion of sites for which the variable (standardized in
all cases except for seed crop) was above average for that site that year and are divided into sites < 50°N and those > 50°N latitude
(variables refer to the former except where noted). Insufficient data for analysis were available for seed production of Pinus > 50°N
latitude in western North America and for breeding densities of hoary redpolls or bohemian waxwings. Listed are the significant
variables, the direction of the relationship, and the level at which significance was achieved. For seed production and breeding
density data, correlations for which P < 0·05 are listed; for weather variables, no variables correlated at the P < 0·01 level and none
are listed. No variable correlated significantly with the eruption index for red crossbills and hoary redpolls
Species
East
West
Evening grosbeak
Previous year crop (+, 0·001)
Previous year crop (Pinus) (+, 0·04)
Breeding density > 50°N lat (–, 0·05)
Previous year crop (+, 0·01)
Previous year crop (Pinus) (+, 0·001)
Current year crop (–, 0·02)
Current year (Pinus) (–, 0·002)
Pine grosbeak
Previous year crop (+, 0·004)
Current year crop (–, 0·01)
Prior year crop (Pinus) (+, 0·02)
Current year crop (–, 0·03)
Current year crop > 50°N lat (–, 0·03)
Breeding density (+, 0·03)
Purple finch
White-winged crossbill
Common redpoll
Pine siskin
Breeding density (+, 0·03)
Current year crop > 50°N lat (–, 0·04)
Bohemian waxwing
Previous year crop > 50°N lat (+, 0·01)
Previous year crop (Pinus) > 50°N lat
(+, 0·02)
Current year crop (–, 0·02)
Previous year crop (Pinus) (+, 0·004)
Current year crop (Pinus) (–, 0·04)
Red-breasted nuthatch
Previous year crop (+, 0·04)
Current year crop (–, 0·03)
Breeding density (+, 0·05)
Previous year crop (+, 0·02)
Previous year crop (Pinus) (+, 0·03)
Current year crop (–, 0·009)
Current year crop (Pinus) (–, 0·002)
Black-capped chickadee
Previous year crop (+, 0·02)
Current year crop > 50°N lat (–, 0·05)
Breeding density (+, 0·001)
Previous year crop (+, 0·003)
Previous year crop (Pinus) (+, 0·04)
Previous year crop (+, 0·03)
Previous year crop > 50°N lat (+, 0·02)
Current year crop > 50°N lat (–, 0·03)
Table 4. Same as Table 1 except that correlations are with the variables as measured in the other geographical region (i.e. for
eruptions in the east, the seed crop is measured in western North America). No variable correlated significantly with the eruption
index for evening grosbeaks, hoary or common redpolls, or black-capped chickadees
Species
East
Pine grosbeak
Purple finch
Red crossbill
Previous year crop (Pinus) (+, 0·05)
White-winged crossbill
Pine siskin
Bohemian waxwing
Red-breasted nuthatch
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
609–620
West
Breeding density (+, 0·001)
Previous year crop (+, 0·01)
Current year crop (–, 0·04)
Current year crop > 50°N lat (–, 0·006)
Breeding density > 50°N lat (+, 0·01)
Previous year crop (+, 0·05)
Current year crop (–, 0·009)
Previous year crop (Pinus) (+, 0·02)
Eruptions of one of the species (purple finches) appear
to be due to high density unrelated to the seed crop,
while we were unsuccessful at determining any correlate
of eruptions in three species in eastern and three species
in western North America.
At least generally, results match expectations based
on the winter diet of the species considered. Major
consumers of conifer seeds include evening and pine
grosbeaks, red and white-winged crossbills, pine siskins
and red-breasted nuthatches (Table 1), eruptions of all
of which are related to the coniferous seed crop either in
the east, west, or both (Table 5). Of the seven species for
which pine seeds make up a substantial portion of the
diet (Table 1), three (evening and pine grosbeaks and
red-breasted nuthatches) correlate with the seed crop of
pines alone either in the same (Table 3) or the opposite
(Table 4) geographical region. Conversely, conifer seeds
do not make up a major portion of the diets of either
JAE516.fm Page 619 Thursday, June 14, 2001 4:11 PM
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Boreal bird
eruptions
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
609– 620
Table 5. Summary of the hypotheses for the causes of winter eruptions by boreal seed-eating birds supported by the results
Hypothesis
East
West
Eruptions occur when a combination of large
seed crops resulting in high densities of birds is
followed by a poor seed crop, forcing relatively
large numbers of birds to wander in search of food
Red-breasted nuthatch
Black-capped chickadee
Pine grosbeak
Pine siskin
Eruptions occur when a combination of large
seed crops is followed by a poor seed crop,
forcing birds to wander in search of food, but no
direct evidence that densities are high
Eruptions occur when the prior year’s seed crop
was unusually large; no direct evidence that
densities are high
Evening grosbeak
Pine grosbeak
Red crossbill
Bohemian waxwing
Evening grosbeak
Bohemian waxwing
Red-breasted nuthatch
Eruptions occur when the current year’s seed
crop is unusually poor
White-winged crossbill
Eruptions occur when breeding densities are
relatively high; cause not determined
Purple finch
Purple finch
No correlate of eruptions
Hoary redpoll
Common redpoll
Pine siskin
White-winged crossbill
Red crossbill
Hoary redpoll
purple finches or hoary redpolls, for which eruptions were
not found to vary in conjunction with the coniferous
seed crop. The two exceptions were common redpolls
and bohemian waxwings, eruptions of which were both
related in some way to coniferous seed crops even though
conifer seeds do not make up a significant fraction of
the diet of either species. This discrepancy between
eruptions and diet is also evident in the weak relationship found between patterns of interspecific synchrony
of eruptions and diet (Lack 1954), particularly in western
North America (W. D. Koenig, unpublished data).
Two potential explanations for the discordance between
eruptions and diet are that synchrony among food
resources may encompass a much larger taxonomic
range of species than currently realized, or that other
factors such as weather may play an important role in
synchronizing eruptions. Our failure to detect any
significant relationships between weather and eruptions
fails to support the latter of these alternatives.
No single factor is apparently behind all eruptions of
boreal seed-eating birds throughout the Northern
Hemisphere, or even within eastern or western North
America. Even within a species, the relative importance
of prior and current seed crops may apparently vary
from region to region (Table 5) and possibly from one
time period to another (Larson & Bock 1986). However,
our results confirm that the seed crop of boreal trees (or
the seed crops of other boreal plants that mast-fruit
synchronously with the trees) are the primary ultimate
factor determining population eruptions of the majority
of boreal seed-eating birds and, in particular, such
eruptions are generally attributable to a combination
of a good seed crop (presumably resulting in high
survivorship and/or high reproductive success) followed
by a poor seed crop the next autumn, forcing the relatively high densities of birds to search for food beyond
their normal range. For four species (red-breasted
Common redpoll
Black-capped chickadee
nuthatches and black-capped chickadees in the east
and pine grosbeaks and pine siskins in the west), nearly
all components of this hypothesis are detectable: eruptions correlate with good seed crops in the previous
year, high densities of breeding birds in the previous
spring, and poor seed crops in the year of the event. For
four additional species in the east and three in the west,
no direct evidence of high densities was found but
otherwise the patterns appear similar, with the caveat
that for red crossbills in the east eruptions are correlated
with western, rather than eastern, seed crops (Table 4).
For several remaining species we were able to garner
support for only a portion of this scenario. However,
for purple finches, eruptions are preceded by high
densities that are apparently unrelated to the seed crop,
while no relationship between eruptions and any of the
variables examined was found for three species in both
the east and the west (Table 5).
Thus, although our results suggest that the seed crops
of boreal trees are a major ultimate factor determining
population eruptions of boreal seed-eating birds they
are apparently not the only factor, at least for some
species in some geographical regions. Future studies,
incorporating large networks of seed crop reports
comparable to CBC counts or the kind of large-scale
volunteer efforts mobilized by Hochachka et al. (1999),
will be needed to further elucidate the specific causes of
boreal bird eruptions in these cases.
Acknowledgements
We thank Sam Droege and Brett Hoover for assistance
with the Christmas Bird Count database and the referees
for their comments. Financial support was provided by
the University of California’s Integrated Hardwood
Range Management Program and the National Science
Foundation.
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J.M.H. Knops
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
609–620
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Received 20 January 2001; revision received 27 February 2001